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The prokaryotes (; singular prokaryote
/proʊˈkæriət/) are a group of organisms that lack a cell nucleus
(= karyon), or any other membrane-bound
organelles. They
differ from the eukaryotes, which have a cell
nucleus. Most are unicellular, but some
prokaryotes are multicellular organisms. The word prokaryotes comes
from the Old Greek pro-
before + karyon nut or
kernel, referring to the
cell nucleus, + suffix
-otos, pl. -otes; it is
also spelled "procaryotes". The prokaryotes are divided into two
domains: the bacteria
and the archaea. Archaea
are a newly appointed domain of
life. These organisms were originally thought to live only in
inhospitable conditions such as extremes of temperature, pH, and radiation but have since been
found in all types of habitats.

Relationship to eukaryotes

A distinction between
prokaryotes and eukaryotes (meaning true
kernel, also spelled "eucaryotes") is that eukaryotes do have
"true" nuclei containing their DNA, whereas the
genetic material in prokaryotes is not membrane-bound. Eukaryotic
organisms may, as in the case of amoebae, be unicellular or, as in
the case of humans, be multicellular. The difference between the
structure of prokaryotes and eukaryotes is so great that it is
considered to be the most important distinction among groups of
organisms. In 1977, Carl Woese
proposed dividing prokaryotes into the Bacteria and
Archaea
(originally Eubacteria and Archaebacteria) because of the major
differences in the structure and genetics between the two groups of
organisms. This arrangement of Eukaryota (also called "Eukarya"),
Bacteria, and Archaea is called the three-domain
system replacing the traditional two-empire
system. A criticism of this classification is that the word
"prokaryote" itself is based on what these organisms are not (they
are not eukaryotic), rather than what they are (either archaea or
bacteria).

It is not surprising that many researchers have
started calling prokaryotic communities multicellular (for example
). Differential cell expression, collective behavior, signaling,
programmed
cell death, and (in some cases) discrete biological
dispersal events all seem to point in this direction. However,
these colonies are seldom if ever founded by a single founder (in
the way that animals and plants are founded by single cells), which
presents a number of theoretical issues. Most explanations of
co-operation
and the
evolution of multicellularity have focused on high relatedness
between members of a group (or colony, or whole organism). If a
copy of a gene is present in all members of a group, behaviors that
promote cooperation between members may permit those members to
have (on average) greater fitness than a similar group of selfish
individuals (see inclusive
fitness and Hamilton's
rule). What to make of prokaryotic communities clearly founded
by many (most likely unrelated) individuals, yet defined by
(apparently) high levels of cooperation, communication, and
coordinated behavior?

It is likely that these instances of prokaryotic
sociality are the rule rather than the exception, a fact that has
serious implications for the way we view prokaryotes in general and
the way we deal with them in medicine. Bacterial biofilms may be
100x more resistant to antibiotics than free-living unicells and
may be nearly impossible to remove from surfaces once they have
colonized. Other aspects of bacterial cooperation—such as bacterial
conjugation and quorum-sensing mediated pathogenicity—present
additional challenges to researchers and medical professionals
seeking to treat the associated diseases.

Structure

Recent research indicates that all prokaryotes
actually do have cytoskeletons, albeit more
primitive than those of eukaryotes. Besides homologues of actin and
tubulin (MreB
and FtsZ) the
helically arranged building block of the flagellum, flagellin, is one of the most
significant cytoskeletal proteins of bacteria as it provides
structural backgrounds of chemotaxis, the basic cell
physiological response of bacteria. At least some prokaryotes also
contain intracellular structures which can be seen as primitive
organelles. Membranous organelles (a.k.a. intracellular membranes)
are known in some groups of prokaryotes, such as vacuoles or
membrane systems devoted to special metabolic properties, e.g.
photosynthesis or
chemolithotrophy.
Additionally, some species also contain protein-enclosed
microcompartments mostly associated with special physiological
properties (e.g. carboxysomes or gas vacuoles).

Morphology of prokaryotic cells

Environment

Prokaryotes are found in nearly all
environments on earth. Archaea in
particular seem to thrive in harsh conditions, such as high
temperatures (thermophiles) or salinity (halophiles). Organisms
such as these are referred to as extremophiles. Many
prokaryotes live in or on the bodies of other organisms, including
humans.

Evolution of prokaryotes

It is generally accepted that the
first living
cells were some form of prokaryote and may have developed out
of protobionts.
Fossilized
prokaryotes approximately 3.5 billion years old have been
discovered (less than 1 billion years after the formation of the
earth's crust), and prokaryotes are perhaps the most successful and
abundant organism even today. Eukaryotes only formed later, from
symbiosis of multiple prokaryote ancestors; their first evidence in
the fossil record appears approximately 1.7 billion years ago,
although genetic evidence suggests they could have formed as early
as 3 billion years ago.

While Earth is the only place in the universe
where life is known to exist, some have suggested evidence of
life
on Mars in the form of fossil or living prokaryotes; this is
open to considerable debate and skepticism.

Prokaryotes diversified greatly throughout their
long existence. The metabolism of prokaryotes is far more varied
than that of eukaryotes, leading to many highly distinct types of
prokaryotes. For example, in addition to using photosynthesis or organic
compounds for energy like eukaryotes do, prokaryotes may obtain
energy from inorganic chemicals such as hydrogen
sulfide. This has enabled the bacteria to thrive and reproduce.
Today, archaebacteria can
be found in the cold of Antarctica and
in the hot Yellowstonesprings.